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ANDERSON COOPER, CNN ANCHOR: Hello.

Welcome to this special report, "81 Minutes: Inside the Greatest Art Heist in History," the greatest in dollars and daring and in just how long investigators have been baffled.

However, that could soon be changing.

Here's 360's Randi Kaye.

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RANDI KAYE, CNN ANCHOR (voice-over): It is a beautiful and priceless collection, hundreds of millions of dollars worth of art housed inside these walls, the Isabella Stewart Gardner Museum in Boston.

Visitors from all over the world come to see these masterpieces. But that's not all they're looking at. They are also looking at a crime scene, in fact, a crime scene from the biggest art heist in history, still unsolved. And this is all that's left.

It is one of the art world's greatest mysteries. Who stole these priceless works of art and how have they managed to elude investigators all over the world for more than two decades? Thirteen works of art stolen, paintings by Rembrandt, Vermeer, Manet, Flinck, sketches by Degas, a Chinese bronze beaker and a finial that set on top of a Napoleonic flag, all taken in the dark of night, all together worth $500 million, $500 million, and not one has ever been found.

This is where it all began the night of March 18, 1990. It was St. Patrick's Day weekend. There was a house party at the building right behind the museum. Some time after midnight, a group of young men left the party and spotted a car with what looked like two Boston police officers inside.

They had no idea these two men weren't really police officers and no clue that just a short time later, these two men would pull off the greatest art heist in history.

RICK ABATH, FORMER ISABELLA STEWART GARDNER MUSEUM GUARD: Come in, clock in. There would be two guards. KAYE: Rick Abath was one of the night watchmen on duty the night of the crime. Until now, he'd never done a television interview about what happened that night. Rick was 23 years old back in 1990. He had been working at the museum for about a year. The guard's shift began at 11:30 p.m., a shift that most nights was uneventful.

ABATH: Boring, sit around, walk around, basically. One guy would go up and start his rounds, and then when he came back, the other, he would get behind the desk and the other person would go up.

KAYE: On this night, things would be far from boring. During the night shift, there is always one guard manning the computer at the security desk by the side door, the employee entrance, which was the only way into the museum at night.

That door was locked. So anyone who wanted to get into the museum would have to ring the buzzer. The guard on duty at the desk could see who was ringing that buzzer through a security camera perched outside right above the door. The camera is in plain sight.

If there was trouble, the guard had just one way of alerting the outside world, a panic button at the security desk. But it wasn't so easy to reach.

ABATH: You know, the same kind of panic button at a bank or something like that. It was up on the underside of the desk. But it was a fairly long desk and the computer that you had to be at to do your job was all the way over to the left, and it was almost all the way over to the right. So it wasn't just within arm's reach.

KAYE: Still, Rick Abath never found any reason to use that panic button, that is, until the night of March 18. That night was unusual from the moment Rick arrived at work.

He learned his usual partner had called in sick. They couldn't find anyone else, so they paired him with a daytime gallery guard.

ABATH: When I got there, you know, my partner was there -- or -- well, I knew him. I didn't know him very well. I knew him from just around the locker room. And he was, I'm told, the only person they could get. So I figured I would take the first round.

KAYE: That first round takes longer than usual.

The fire alarm goes off for no apparent reason. So does another alarm on the fourth floor. After all that, Rick returns, and the gallery guard takes his turn making the rounds. The clock reads 1:24 a.m., and Rick is alone at the guard desk. Suddenly, the night takes a dangerous turn.

ABATH: Cops rang the doorbell. And I could see them on the outside camera. I could see them walking down the street. I thought they were just clearing drunks off the street, because there were a lot of drunken people in the street that particular night.

But then they stopped and they buzzed the buzzer. And I just kind of leaned over to the intercom and said, yes? And they said, Boston police, we got a report of disturbance on the premise. So I buzzed them in.

KAYE: That decision to buzz them in is something Rick Abath has had to live with for the past 23 years.

Anthony Amore is the security director for the Isabella Stewart Gardner Museum. He's also an active investigator on this case. He says back in 1990, no one was allowed into the museum after hours, not even police officers. But Rick, the security guard, said he'd routinely buzz in museum employees after hours, even the museum director.

ABATH: I was never verbally told that, to the best of my knowledge. I know -- I'm sure there was a policy written somewhere. I'm sure it was -- but it wasn't the culture of the place for the most part. People came in and out of there fairly regularly. At least once a month, we'd let someone in.

KAYE: So it was not unusual for Rick to hear that buzzer go off. On this night, Rick says he had no doubt it really was the Boston police at the entrance.

ABATH: They were standing there in their pants and coats and badges and they looked like policemen, buzzed them into the museum.

And one of them came right over to my desk. And one of them kind of stood in the alcove right there, just looking around, which didn't seem particularly odd to me. And he came up to me, the one guy, and they asked me -- he asked me if I was alone. And I said that, no, my partner in was off doing a round. He said, get him down here. So, I called him on the radio.

KAYE: Within seconds, Rick's partner joined him at the security desk.

ABATH: The cop that was dealing with me turned to me and said, don't I know you? Don't I recognize you? I think there's a warrant out for your arrest. Can you step out from behind the desk?

And I'm sitting there and I looked the way I look. I don't look substantially different, other than I didn't have any white hair then. And, you know, I had my Berklee College of Music tie-dye on. I did have the Gardner security shirt on, but it was over my tie-dye and it was unbuttoned.

And I was like, God, this -- OK, I know how this looks. It looks like I put these guard clothes on over my street clothes.

KAYE: Here, Rick makes another grave mistake. He steps away from the security desk and away from the panic button, his only way to contact the outside world, his only way to prevent what was about to happen.

ABATH: So I came out from behind the desk and I gave him my driver's license and I gave him my security I.D. and I think I was kind of singsongy, sarcastic about it, like, I really work here. And he said, up against the wall. There's a warrant out for your arrest.

And so I put my hands up against the wall and he handcuffed me. He didn't frisk me. He just put a cuff on me. And that's when I heard my partner behind me shouting out, officers, why are you arresting me? Why are you arresting me? And I was just like, it's over. It's done.

KAYE: In a matter of minutes, the two thieves had both night watchmen completely under their control.

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KAYE: After two thieves dressed as Boston police officers gained access to the museum, they had both night watchmen completely under their control within minutes. There was no time for either of the guards to call for help.

And it seems the thieves knew it. The priceless collection of art in the museum is now theirs for the taking. It's about 1:30 in the morning, and it's time for the thieves to get down to business.

ABATH: He finished cuffing me, and he cuffed my partner and very dramatically said, gentlemen, this is a robbery. You know, just do what we say and you won't get hurt. And I said, we don't get paid enough to get hurt. That kind of defused the situation a little bit.

KAYE (voice-over): Rick Abath laughs about it now, but as soon as he was tied up by the two thieves, he realized how vulnerable he really was.

ABATH: I was panicking. I didn't realize I was panicking. But I was completely panicking.

KAYE: The thieves lead Rick and his partner down to the basement to differ different areas. Rick is taken to the boiler room and cuffed to an electrical box. His eyes and mouth were duct-taped and he feared for his life.

ABATH: I was afraid that they were going to set the place on fire after they were done. That was my -- maybe because I was right across from the boiler. But that was -- my predominant fear was, God, I hope they don't burn the place down.

KAYE: He couldn't see, couldn't hear. And he had no idea what was happening upstairs. All he knew was that his museum was unguarded and about to be robbed.

It all happened so fast, he never had a chance to hit the one panic button by the guard desk. He knew no one was coming to help.

Did the thieves know that as well? It appears they did since they were in no rush to get out.

AMORE: The path is interesting. They took the guards after they handcuffed them and taped them and brought them into the basement. About 24 minutes elapsed, though, before they -- we see them again.

KAYE: Motion detectors placed throughout the building picked up their trail for nearly an hour-and-a-half. But that didn't matter. Those motion detectors weren't connected to police outside. They only alert the guard sitting at the computer by the entrance, a computer that was now unmanned.

AMORE: It's in this hallway where we see the first motion detectors go off. So, that's how we know that it was 24 minutes. So, it is about 1:48 and they're walking down this hallway together and they enter the Dutch Room.

KAYE (on camera): Which is right there.

AMORE: Exactly. And from the Dutch Room, they took six pieces.

And that's where...

KAYE: Including the Rembrandts.

ABATH: The Rembrandt -- the three Rembrandts, the Vermeer, and the Flinck and the Chinese vessel. And that's where -- in terms of dollar value, that's where the lion's share of the theft occurred.

KAYE (voice-over): These three Rembrandts, this Vermeer, this landscape by Govaert Flinck all had to be taken down from the walls in the Dutch Room. The real work had begun for the thieves. But as they get ready to remove Rembrandt's Storm on the Sea of Galilee, his only seascape, a high-pitched alarm sounds.

Pulitzer Prize-winning reporter Stephen Kurkjian has investigated this case for "The Boston Globe" for decades. He first interviewed Rick Abath about the night of the crime for "The Globe" and Abath told him this alarm was designed to keep visitors from getting too close to the Rembrandt.

STEPHEN KURKJIAN, "THE BOSTON GLOBE": That seascape, even if you look at prints of it now, images of it now, you will see a vision, an etching of Rembrandt himself. Art experts, art specialists, common folk knew that, and they would come up and they would point their finger close to, point out the image of Rembrandt. And if they got too close, then the alarm would sound.

KAYE: Kurkjian says the thieves then smashed the alarm.

KURKJIAN: That was really a compelling little detail to me, because it seemed like that worked, that, you know, if you got too close, the sound would go off. And as I went through the rest of the security that was in place at the museum, nothing else was there to guard the museum's valuables.

KAYE: Like the motion detectors, this alarm was not connected to the outside world. But did the thieves know that as well? Because they didn't pack up and leave at that point. They continued on with their crime, and they took their time.

One thief stayed behind in the Dutch Room and carefully removed some of the paintings from their matting and frames, although two of the Rembrandts were simply cut out of their frames. The other thief headed out of the room, back down the hallway.

AMORE: Same path backwards, goes through the Early Italian Room, the Raphael Room, all the while passing incredibly priceless art, famous art, Raphaels, important Chinese pieces, Fra Angelico, and walked back to the short gallery where the thief takes five sketches Degas, and a Napoleonic finial from a top of a flag that Napoleon's first regiment carried.

Throughout his actions here in the short gallery, he's going back and forth about a half-dozen times, again, passing things that any art expert would say, my God, these are two Raphaels, small and portable. Why wouldn't you take those? It's a great mystery to the theft.

KAYE: Another great mystery is this painting by Edouard Manet. The oil painting was taken from the Blue Room on the first floor.

AMORE: It hung right below Manet's portrait of his mother. It was about eight-by-10 and it was in a gold gilded frame and it was here.

KAYE: Even though motion detectors picked up the thieves' trail by the entrance on the first floor and all throughout the second floor, there are no records of anyone entering the Blue Room.

Rick Abath says in the time he worked at the museum, the motion detectors never failed. But he also says it wasn't impossible to avoid the detectors. In fact, he knew exactly how to do that. It was a game he used to play at night to help pass the time on his rounds.

ABATH: You would walk in all kinds of crazy ways going -- some places, you would have to go around the perimeter of the room or you would have to go right through the center, you would have to duck down, or you would have to kind of squat and waddle, and do all these different stuff.

KAYE: It's something straight out of the movies, like this scene from "Ocean's 12."

The motion detectors at the Gardner Museum were not nearly as complicated as that. Still, the thieves still seemed to know how to evade them. But another question emerges: Why would they bother doing so only in the Blue Room, where just one painting was stolen? Unless they weren't the ones who actually took the Manet out of the Blue Room that night.

The thieves made off with the printed records of the motion detectors, though they left the computer, which saved all of the information on its hard drive intact. Meanwhile, all of this time, Rick Abath is waiting in the pitch dark, scared to pieces in the museum basement.

As he waited for something to happen next, he suddenly realized someone was staring at him.

ABATH: Either through sweating or struggling or both, at some point, the duct tape had slipped down.

So I could see a little bit over the duct tape, kind of. And at one point, somebody did come and check on me. Somebody was standing at the end looking down at me. But they didn't say anything. And I -- it spiked me up again. I was freaking out again, but trying not to, like, oh, my God, I have to pretend I can't see this person, because that would definitely not be good if they realized I could see them.

KAYE: Rick says he had lost track of time and wasn't sure exactly when the thief came to check on him. That was the last he'd see of them. Their job was done.

But remember the video camera perched outside the door where they entered? The thieves must have known it was there and must have known where it was hooked up to, because before they left the museum, they made one last move to be sure nobody ever recognized them.

AMORE: There was a point where they kicked open a door, pried open a door, broke it pretty badly, and took the videotape we had, the only evidence of what they looked like, from the security director's office.

KAYE: At 2:41 a.m., the door to the museum opens and closes, and then opens and closes again four minutes later. It must have taken the thieves those two trips in and out to load up the art. Then, just as suddenly as they arrived, the thieves were gone.

KURKJIAN: Once they leave, they're never heard from again.

KAYE: Rick Abath's fear that the thieves were going to burn down the museum prove to be unfounded. The museum was left largely unharmed. But 13 of the greatest works of art, worth about half-a- billion dollars, were missing.

Rick says he just remembers waiting and waiting and waiting. Police found him the next morning.

ABATH: The police came around the corner with flashlights, and the guy seemed surprised and screamed out, whoa, we have got another one. And I was just, like, cut me off this F-ing box, because my hands had fallen asleep a long time ago.

KAYE: Rick was relieved to be found and to be alive. But he knew almost immediately that he was a suspect.

ABATH: I knew I would. I mean, I opened up the door, you know. I mean, once I sat down with the FBI, I think the first thing I said was, what do you want to know? Because I knew. I mean, well, I'm the guy who opened up the door. They're obviously going to be looking at me.

The FBI certainly was looking at him. Was it an inside job? How else could the thieves have pulled this off? And who else did the FBI suspect?

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KAYE: Dawn, and the damage was done. When security guards showed up for their regular morning shift and rang the buzzer, as they always did, they knew immediately something was wrong. They called their supervisor and soon the Boston police and the FBI were on the scene. Reality sunk in; $500 million worth of art was gone.

(voice-over): There was no trace of the thieves. Authorities got an idea of what the bad guys looked like from the two night watchmen, the only ones to see the thieves up close. But it all happened so fast. They were tied up and blindfolded within minutes.

Watchman Rick Abath gave this description of the guards to a sketch artist.

ABATH: So, the guy who was dealing with me was kind of taller and skinny and was wearing his gold-framed, like, round glasses, if I remember correctly. And he had a mustache. And I remember before he arrested me, that it looked really greasy. I remember thinking that was really -- he was using some funky kind of wax on that thing or something on that. It was probably a fake mustache.

KAYE: But the descriptions from Rick and the other guard didn't satisfy the FBI. Even Rick admits the sketch they produced didn't really look like either of the two men.

ABATH: I remember at the time thinking, there's no way they're going to catch these people from this.

GEOFF KELLY, FBI BOSTON: Unfortunately -- and that's one of the frustrating aspects of this case is the descriptions that were given were very vague, very generic.

KAYE: Geoff Kelly from the FBI's Boston office is the lead agent in this case. Without a good description and virtually no other public information about the thieves' identity, the investigators begin to focus their attention on the museum employees.

KELLY: These guys had a very -- they had a level of comfort in that museum that really points to the fact that, if it wasn't an inside job, they definitely had inside information.

KAYE: Both the FBI and the museum security director are stuck on the fact that the thieves spent 81 minutes inside the museum. Anthony Amore says that is eight times as long as the typical art theft.

AMORE: There was a covert button, just like you would see in a bank, and the thieves knew somehow that that hadn't been hit as evidenced by how much time they spent in the museum.

KAYE (on camera): They were not at all concerned, 81 minutes, that the police were on their way.

AMORE: Clearly not concerned that the police were coming. So how they knew that the guard didn't hit that alarm is a mystery to me.

KAYE (voice-over): But did the thieves really know the alarms had not been set off? Or could that be why they waited a while to start dismantling the art? Remember, motion detectors didn't pick up the thieves' trail until 24 minutes after they entered the museum.

ABATH: Apparently, there was a half-hour between -- 20 minutes between when they tied us up and when they actually headed into the galleries to start doing it.

And I have seen a lot of people questioning like, well, what were they doing? And I'm thinking, they were probably waiting to see if I did press the panic button. They were probably waiting to see if the cops were going to show up.

KAYE: According to the FBI, nearly nine of every 10 museum heists have an inside component.

Rick Abath, the guard who let the thieves in, was interrogated for days. He also took lie-detector tests. He was never charged with anything. And when we talked with him, he maintained he had nothing to do with the heist.

And what about the regular night watchman who called in sick that night? Rick remembers, after he buzzed the thieves into the museum, they asked him if he was alone.

ABATH: I wonder if that -- are you here alone? Because there were provisions for a single guard to be here. I have wondered if "Are you here alone" meant "did they get anybody to come in?" But I don't know.

KAYE: But that night watchman, the one who called in sick, was never charged, either. In fact, no museum employee has ever been charged with anything in connection with the crime. Though investigators say no one has been completely cleared, either.

So with no apparent connections with museum employees, who else could have been involved? Notorious art thief Miles Connor was already a known figure among art thieves in Boston. In 1975, he stole a Rembrandt from a Museum of Fine Art. But in 1990, at the time of the Gardner theft, he was in jail, and no known connections were ever found.

Mobster David Turner from Boston also was considered a possible suspect, with various publications pointing out the resemblance between the FBI sketch and Turner. But no definitive evidence linking Turner to the heist has ever surfaced.

Boston crime boss Whitey Bulger was also rumored to be involved, with speculation he stole the artwork in order to smuggle it over to the IRA, the Irish Republican Army.

GEOFF KELLY, FBI BOSTON: Certainly, if you're looking at Boston, which has a huge Irish population, and in -- it's not unusual or you wouldn't be incorrect in assuming that there might be some connection to the IRA. The IRA Has done it before, where they've taken paintings and basically ransomed them back for money. They've done this in the past. And because of the connection to Boston, it's not -- it's improbable.

But, again, like many of the other theories, there's been nothing concrete that we've developed over the years.

KAYE: The Justice Department has publicly disavowed any link between Bulger and the Gardner Museum.

And so the search for the thieves continues. And, at the same time, the effort to recover priceless works of art.

CHRISTOPHER MARINELLO, EXECUTIVE DIRECTOR, INTERNATIONAL ART LOSS REGISTRY: This is the creme de la creme of art recovery.

KAYE: Christopher Marinello is an investigator and executive director of the International Art Loss Registry, the world's largest database of stolen art. He's been waiting to see if any of the stolen art from the Gardner Museum has come up for sale anywhere in the world. After all, if the thieves are after money, they will try and sell the art.

So Marinello watches and waits.

MARINELLO: We'd probably be the first ones to know about it. We do explore a lot of leads. We pass on those leads to law enforcement and let them run with it.

KAYE: Investigator Robert Whitman says when it comes to stealing art, the actual theft is the easy part. Once they have the goods, they need to know how to sell it without getting caught.

ROBERT WITTMAN, ART CRIME INVESTIGATOR: That's the next step. And if they're going to sell it, it's like a car. It's not like that. The market for these artworks is very small.

KAYE: According to Wittman, nearly all stolen, high-end artwork is eventually recovered.

Investigators continued with their search, chasing down countless tips and leads. Then, in 1994, a possible breakthrough. That year, the museum director received a mysterious, un-signed letter post- stamped from New York. The anonymous writer said he had access to the stolen art and would barter them back for 1 percent of their value.

STEPHEN KURKJIAN, INVESTIGATIVE REPORTER: The writer asked for two things. One was that the investigation stand down. What he meant by that was he was worried that those people who would facilitate -- help him facilitate the return, would get arrested. And if he couldn't assure them fully that no one was going to get arrested here, that -- then the deal was off.

And the second thing he asked for was that an indication come from the museum that they were willing to engage the FBI and go for serious negotiations here.

KAYE: The indication the writer asked for was this. The No. 1 printed in the "Boston Globe" Sunday currency table next to the Italian lira. The museum complied and printed the secret signal and then they, and investigators, waited.

KURKJIAN: A short time later, they received a second memo. "Thank you very much. I appreciate the willingness to engage. I saw the one. However, I must tell you that we are still worried that they have not stood down." That the FBI has not stood down.

KAYE: The hot lead went cold fast. The museum never heard from that person again. But another lead was surfacing, this one pointing straight to Hollywood.

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KAYE: What happened inside this museum nearly 23 years ago baffles investigators to this day. They haven't been able to catch the thieves, and $500 million worth of art seems to have vanished without a trace.

KURKJIAN: I like to say it was Boston's last, best secret.

KAYE (voice-over): Boston's last, best secret. Who stole the 13 works of art from the Isabella Stewart Gardner Museum, and where are they now? It is a question that continues to puzzle museum security director Anthony Amore.

(on camera): What it is about this case that keeps you up at night, that just doesn't sit right with you?

ANTHONY AMORE, FORMER MUSEUM SECURITY DIRECTOR: There were lots of quirky things about it. Every time you turn around, there's a different, interesting fact that you find.

KAYE (voice-over): One of the biggest questions for Amore: Why did the thieves steal what they did?

Remember the path of the thieves on the second floor? When they went from the Dutch Room to the Short Gallery, they bypassed valuable works of art that were small and portable and worth a lot more than some of the other art they stole.

KURKJIAN: The two big Rembrandts and the Vermeer, those three pieces account for 90 percent of let's call it 500 million, 400, 500 million, whatever the mix, of the value. KAYE: Another big question, why did the thieves bother taking the golden finial that sat atop of a Napoleonic flag? At first, it seems they attempted to steal the flag itself. But it proved to be too difficult to take down.

KURKJIAN: They wanted very badly to get that flag. And that's always seemed to me to be the clue that is the most interesting. And I know the FBI has spent a lot of time trying to figure that out. Visiting, talking to, associations who are involved with Napoleonic memorabilia to see was there a bounty for one of these banners?

KAYE: Why did the thieves want that flag so badly? Is it possible they were given a specific list of artwork to steal? By a collector who especially loved those 13 works of art?

That brings us to another theory. One that has been made popular by Hollywood.

UNIDENTIFIED MALE: One million dollars, Mr. Bond.

KAYE (on camera): Do you think it's possible that they're in some private gallery owned by some eccentric billionaire?

WITTMAN: No. No.

KAYE: Like Dr. No type character?

WITTMAN: The whole idea that, you know, a collector is holding onto stolen works of art all came from that movie "Dr. No." The 007 movie, James Bond movie, 1962, where James Bond is going through Dr. No's lair. He looks over and he sees a painting hanging on the wall. OK? And that painting had been stolen the year before. That actually did happen.

In 20 years, the FBI and five years since then doing these investigations, I've never run into a collector who had million-dollar paintings that were stolen.

KAYE: Anthony Amore doesn't buy the Dr. No theory, either.

AMORE: My gut instinct is that it's not far. Typically, when art is stolen, it doesn't get moved very far. If I had to guess, I would think it's still in New England.

KAYE: Since 2003, the U.S. attorney's office has offered complete immunity to anyone who came forward with information on the paintings. And the museum is offering a $5 million award. That's a lot of money on the table. So why hasn't anyone come forward?

KELLY: The other alternative, and it's a strong one, is that whoever has them just is waiting for the right time. And it's not a question of the $5 million or immunity. It's a question of this individual might get jammed up on some other unrelated crime and he or she might feel that this is their trump card to get out of prison.

KAYE: Stolen art can be a get-out-of-jail free card for criminals, which is why the notorious Whitey Bulger was looked at with suspicion for a while. And more recently, just last year, authorities searched the home of alleged mobster Robert Gentile, who was in prison on drug and weapons charges. They found no sign of the artwork, and Gentile remains behind bars.

There are some who don't completely buy this theory. Former FBI Agent Robert Wittman is one of them.

WITTMAN: My personal opinion, just from experience, I think some of it has been dispersed in different places. And I think it's still possible that -- it's possible that the Vermeer and the Rembrandt could be in western Europe.

KAYE: Wittman's theory is based on a 2006 lead the FBI called Operation Masterpiece.

WITTMAN: It was resurfacing somewhere in the south of France at that time. They were being offered for sale for approximately $30 million at that point.

KAYE: Hoping to crack the case, Wittman went undercover as an interested buyer. And although the FBI did recover over $65 million worth of stolen art during that operation, not a single piece turned out to be stolen from the Gardner Museum.

But that doesn't mean that the people offering the artwork didn't have it. In fact, they talked specifically about selling the missing Rembrandt seascape and the Vermeer.

WITTMAN: The case was close to possibly recovering some pieces. The reason I say that is because I've worked for this almost 20 years at that point. Through my experience, I think that, you know, these people were talking in reality. They were talking about the things they really had.

KAYE: But that was nearly seven years ago. And Wittman fears the paintings could be anywhere by now.

For art crime investigators, this Gardner artwork has become the holy grail.

MARINELLO: I think that everybody that knows about art theft and art recovery knows these 13 items. They've been widely publicized. I think these pieces are unsalable in today's marketplace.

KAYE: If that's true, then what will become of them? How will investigators ever find them? With offers of immunity ignored and the $5 million reward disregarded, the mystery only deepens.

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KAYE: The Isabella Stewart Gardner Museum is considered one of the finest institutions in Boston. So when two men posing as Boston police officers made off with 13 works of art, it became priority No. 1 for investigators working the case. Their goal has always been to return the stolen artwork to where it belongs. But even if they do one day, what may always haunt them is how the thieves knew the museum was so vulnerable.

(voice-over): For 23 years, these empty frames have hung on the walls of the Isabella Stewart Gardner museum. But they are more than just evidence of a remarkable crime. They are a sign of hope, placeholders for when the paintings return.

All these years, one thing investigators haven't been able to figure out, besides where the artwork may be, is how did the mastermind behind this heist know security was so thin at the museum? Just two night watchmen, one panic button, motion detectors that could be dodged and one security camera outside. Did the thieves have help from inside? Would they have been able to pull it off without inside information?

Suspicion still surrounds the men on guard that night. Night watchman Rick Abath said, while he doesn't know of anyone in the museum who would have knowingly tipped the criminals to the lax security set-up, he admits it was a topic he and other guards spoke about often.

ABATH: We talked about in terms of if the place got robbed or when it would get robbed. It -- the security was very much a concern. We knew there were no alarms on the paintings. There was no alarms that went outside the place other than the panic button. It was definitely a concern.

KAYE: A concern that he and others would speak about in public.

KURKJIAN: If you're worried about vulnerability in the security system, you can sell that. You -- and that's what I think happened here. Someone who had some interest in art made known to the criminal underworld, to a gang that he or she was familiar with, the information that this museum's security system is abysmal. And this is the components of it.

How that person learned of it, whether he or she had a relationship with one of the guards, had picked it up at barroom conversation, that's yet to be determined.

KAYE: To this day, no museum employee, including Rick Abath, has been fully cleared of the crime. Investigators are also still trying to determine why the thieves took what they did. And a more tantalizing question. The treasures they left behind.

AMORE: Two doors down was a Michelangelo. They walked about a half dozen times by Raphaels. They walked by a Botticelli, a 500- year-old Botticelli. To think these things were bypassed, when they had all of this time, ample opportunity to take them.

KAYE: And why the golden finial? A seemingly random object to steal, unless there's a connection to the lead investigated by the FBI back in 2006. That tip originated in the south of France, which could mean the mastermind behind the crime was interested in acquiring Napoleonic artifacts as well as the art.

Still, so many questions, so few, reliable leads. But the trail may not be as cold as you think.

MARINELLO: We do find that there's those two polar extremes. One is a very quickly after the theft, we'll locate the work because it's being offered for sale by a criminal or a handler. Or art will go underground, and we won't see it for a decade. Very little in between.

KAYE: Investigator Christopher Marinello recently recovered a Matisse that was stolen 25 years ago. So he has hope the Gardner paintings will resurface. So does Anthony Amore, who has worked tirelessly, along with the FBI, to get the artwork back.

(on camera): Do you think you're close?

AMORE: I do. I do. I've been saying that for a couple years now. And people are going to start to wonder about me. But it's a slow process. We liken it to a needle in a haystack. But we're trying to make the haystack smaller.

KAYE (voice-over): And it seems that haystack is finally getting smaller. Just this week, 23 years after the heist, the FBI announced they have identified the thieves who pulled it off. Agents refused to reveal the thieves' identities, citing an ongoing investigation. They would only say the thieves were part of a criminal organization based in New England and the Mid-Atlantic.

And because it's been so long, the statute of limitations has expired. The thieves can no longer be prosecuted.

And the stolen artwork? All 13 pieces remain missing.

UNIDENTIFIED MALE: We have reason to believe it's likely that the artwork has changed hands several times.

KAYE: Investigators are also offering the possibility of immunity to anyone who has the artworks. They just want the artwork back in good condition. Because if the paintings are stashed somewhere, perhaps hidden in an attic or behind a basement wall without the right temperature or right humidity, they will be ruined, if they're not already.

It's a scenario investigators hate to even talk about, because it would likely mean these 13 masterpieces are gone forever from public view.

WITTMAN: We all have a complication to maintain this material for our children. And when something like that happens, and if it's destroyed, well, we'll be letting our children down.

KURKJIAN: It's stolen from the museum, it's stolen from Boston, it's stolen from America, it's stolen from the world.

ABATH: I was a victim. The museum was a victim. The city of Boston was a victim.

MARINELLO: It's a huge, huge theft. And we hope, one day, it will be a huge, huge recovery.

KAYE (on camera): If that day ever comes and the half a billion dollars' worth of art was returned here, there would be a celebration. Only then will we finally have some answers as to how these two anonymous thieves were able to flawlessly pull off the greatest art heist in history and then vanish with the artwork into the dark of night without a trace.